204 J. W. SPENCER^ M.A.^ ON THE GEOLOGICAL 



etc., which were accumulated in the district now elevated to 

 form the island. As these deposits lie beneath old Tertiary 

 formations, their origin cannot have been later than the early 

 part of the Tertiary period. 



Passing by the minor changes, the general condition of the 

 region in the earlier Tertiary period was one covered by the 

 sea, at first with the accumulation of clastic strata derived from 

 the remains of the older volcanic rocks. These were succeeded 

 by white limestones and marls containing fossils which show 

 their age not to have been later than the Oligocene period, by 

 the close of which time the general construction of the 

 Antillean plateau was formed ; to be greatly denuded and 

 carved into its present broader outlines during the long 

 Miocene-Pleiocene period. During the earlier part of the land 

 history, its elevation appears to have been considerable, but the 

 altitude was gradually reduced so that about the end of the 

 Pleiocene days its surface was reduced in area so as to leave 

 only the present chain of islands somewhat smaller than 

 now. During this long period of land surface, North and 

 South America were bridged by a now sunken plateau. 

 Throughout this long era we are not certain that volcanic 

 activity was energetic throughout the region, and the dif- 

 ferentiation of the many tufaceous formations of the more 

 western islands needs further elucidation. 



After the subsidence about the close of the Pleiocene 

 period, the whole region rose to form a very high table-land, 

 whose elevations as shown among the Windward Islands them- 

 selves, must have been from 3,000 to 6,000 feet, but from the 

 evidence among the Bahamas and off the coast of the North 

 American continent, it must have exceeded two miles, at a 

 time which corresponded to the earlier Pleistocene days. 

 This elevation seems to have been a sufficient cause of the 

 Glacial period. Then the rapidly descending streams carved 

 out the canyons which became great deep river-like valleys 

 now submerged beneath the sea. 



Upon the changes of level which carried down the land of 

 the later Tertiary days below sea-level, volcanic activity after 

 the long rest was renewed as we have already seen, and the 

 volcanic ridges were in part upheaved and further built up by 

 the eruptions mostly before the mid-Pleistocene period, which 

 reduced the area of the islands to sizes smaller than now. In 

 short, the great chain of volcanoes belongs to the late Tertiary 

 and post-Tertiary days, being coincident with the great changes 

 of level of land and sea. 



